Aqueous metalworking fluids are well known and widely used because of their economic, environmental and safety advantages over nonaqueous metalworking fluids. Aqueous MWF have very low flammability and with the ever-increasing cost of petroleum-based products, their economic advantage over nonaqueous MWF continues to grow. Moreover, aqueous MWF do not carry the obvious environmental burden, at least to the same degree, of use and disposal that petroleum-based fluids carry. However, beyond these safety, economic and environmental advantages, aqueous MWF must also exhibit other properties, e.g., not stain the workpiece and stability during storage and use.
Aqueous MWF comprise mostly water, typically in excess of 95, often in excess of 97, weight percent (wt %). Water tends to stain certain ferrous and nonferrous workpieces, particularly aluminum, under typical metal working conditions, especially if the MWF has a relatively high pH, e.g., above 9, which is typical of many aqueous MWF. Certain materials, however, can be incorporated into the aqueous MWF to impede the staining of the workpiece, e.g., sodium silicate and phosphate esters, but these materials often have deficiencies of their own. For example, silicates tend to plug the ultra-filtration membranes frequently used in the recycling of the MWF, and phosphate esters are subject to relatively rapid bacterial degradation.
Accordingly, the metal working industry has a continuing interest in identifying additives and aqueous MWF formulations that reduce or eliminate the staining of a metal workpiece, particularly a nonferrous metal workpiece like aluminum, during and after a machining operation. Moreover, the industry, particularly small and medium size job shops, have a continuing interest in such additives and formulations that are effective on both ferrous and nonferrous metals because it allows them to avoid the need to purchase and inventory multiple aqueous MWF.